Ongoing global conflicts have shown that there is a need for a low-cost solution to the problem of detecting and tracking incoming fire. Currently no system exists to provide troops with an effective automated method for detecting rocket-propelled grenades (RPGs) or other incoming fire. In some cases, for example, helicopter pilots do not even know they are being shot at, either by small arms fire or RPGs.
While passive defenses such as reactive armor or other artifices such as chicken wire have been suggested to countermeasure the RPGs, they are not effective for this purpose.
Moreover, it will be appreciated that the entire point and shoot scenario must be accomplished, for instance, within 150 milliseconds. This is because typically there are only 150 milliseconds from the time that the trigger is pulled on the RPG until impact.
Currently available warning suites tend to be too expensive to deploy on every vehicle. Consequently, only high value assets are protected by these systems. What is therefore needed is a standardized, inexpensive, active warning radar in which every vehicle could be outfitted with, at minimum, a warning device. Additionally, interfacing the radar to an appropriate countermeasure would provide for a modular and flexible outfitting methodology.
Once the range, direction and velocity of an incoming ordnance has been determined, there needs to be a suitable countermeasure that can be appropriately aimed and fired. One such system involves the use of a so-called shotgun in which a pattern of pellets is projected towards an incoming RPG. The requirements of such a system are severe in that one must be able to ascertain the trajectory or path of the incoming RPG and to be able to project out a pellet pattern that is sufficiently dense to countermeasure it. If the shotgun is mispositioned, then the pattern will miss the RPG altogether. Also important is the fact that, since the pellets disperse out in a cone, it is required that the pellets intersect the incoming RPG at an optimum range to assure optimal pellet density and cross section. If the pellet cloud intersects the incoming RPG too far away, then the pellets will have dispersed too much to guarantee an RPG kill. If the range is miscalculated such that the shotgun is fired when the RPG is too close, then the cone is so narrow that any slight aiming error will cause the narrow pellet cloud to miss the RPG.
Typical range accuracy goals for shotgun-based systems are on the order of one-half a meter, whereas typical velocity measurements require one-half meter-per-second accuracies. Moreover, angle-of-arrival accuracies need to be on the order of 0.8 degrees. Note that eight-tenths of a degree accuracy is difficult to attain.
It is thus necessary to be able to provide a system in which the angle of arrival can be accurately ascertained to within 0.8 degrees and wherein the range of the RPG can be ascertained within one-half meter.
It might be thought that one could use infrared (IR) detection techniques to detect the plume of the RPG after it is fired. However, upon reflection it will be appreciated that the bloom on the focal plane array of an IR detector is much too large to be able to provide the aiming accuracies required.
In the past, systems have been suggested to provide the angle of arrival and range measurements based on a pulse Doppler approach. However, this approach leads to a very expensive implementation. This is because the times of arrival of the pulses are used to determine range, which requires very tight timing requirements and very high throughput digital electronics. Additionally, the analog-to-digital converters required in pulse Doppler systems must operate at the pulse rate of the radar, which can be several orders of magnitude above simpler approaches. Thus, traditional pulse Doppler radars that can measure the required parameters use expensive components and require large spectral bandwidths.
Another approach to determining range and angle of arrival of an incoming ordnance involves so-called two-tone monopulse radars. As described in U.S. Pat. No. 2,907,999 issued to T. L. Wadley and U.S. Pat. No. 5,402,129 issued to Robert C. Gelner et al., two-tone monopulse CW radars have been used in the past to provide range and angle of arrival. In each of these systems, CW radar signals of two different frequencies f1 and f2 are detected and are separated into Sum and Difference channels. Because certain amplitude and phase relationships exist between the Sum and Difference beams of the two frequencies, range and angle of arrival can be ascertained. While these systems provide estimates of angle of arrival and range, their accuracy is too poor to support the types of aiming accuracies required to shoot down an RPG or other incoming projectile.
The reason that the prior two-tone monopulse CW radars have been unable to deliver the required accuracies derives from the fact that not all of the information that is developed in the Sum and Difference channels is used. Moreover, two-tone monopulse digital processing techniques in the past have first analyzed angle of arrival and then have used the results to determine range. The sequential processing in essence discards a fair amount of available information from the radar returns and takes up valuable time.
In one battlefield scenario, one typically uses IR detectors or bolometers, which have a 30-degree field of view that can detect the firing of an RPG through detecting the plume associated with the launching of an RPG. The purpose of using such a bolometer detector system is first and foremost to detect the launch of an RPG and secondly to be able to provide coarse coordinates for the aiming system for the shotgun. In order to provide a 360-degree ring of protection, for instance, for a HMMWV or other type of small vehicle, one would need to use multiple bolometers or cameras. These bolometers use IR focal plane arrays and take about 30 milliseconds in order to obtain the coarse angle of arrival. One therefore is left with 120 milliseconds to be able to reposition the gun and fire it. Next, one must use some type of system to refine the angle of arrival and to detect the velocity and calculate the range of the RPG.
When considering monopulse radars, the way that monopulse two-tone radars work is to provide two receive antennas, for instance, right and left for azimuth, and to form a Sum beam and a Difference beam. For elevation, two additional orthogonally oriented antennas are used. In either case the beams are added and subtracted to provide two channels. In order to obtain the angle of arrival, one divides the amplitude of the Difference beam by the amplitude of the Sum beam, with the ratio correlated to angle of arrival. However, this kind of angle of arrival measurement has a number of problems.
First, if there is more than one target in the beam, the system simply does not work. More importantly, one does not necessarily even know of the existence of multiple targets. The above monopulse radars are thus typically used in air-to-air situations where no other targets are visible.
Also, if one is off boresight more than, for instance, 4.5 degrees, one obtains erroneous answers. If the true angle is outside the plus or minus four-and-a-half degree limit, one still obtains an answer, but this answer indicates that the angle of arrival is within the four-and-a-half degrees, thus presenting an ambiguity that cannot easily be resolved.
Moreover, as mentioned above, calculating angle of arrival and calculating range has involved two separate calculations. The problem with this approach is that two separate calculations introduce calculation errors. As will be appreciated, systems that require sequential calculations have inherent accuracy limits.
Most importantly, by using prior art processing of the Sum and Difference signals from one frequency only, one throws away data that exists in the data returns which, if used, could improve the angle of arrival and range accuracies.
If one could ascertain a way to combine the range and angle of arrival measurements so as not to throw away available information, then one could obtain twice as good angle of arrival estimates and twice as good range resolution.
By way of further background with respect to two-tone monopulse radars, it is noted that, in these calculations, if one is using the Sum f1 and the Sum f2 in obtaining the phase Difference to estimate range, one is actually throwing away available information associated with Diff. f1 and Diff. f1. Moreover, if one, as in the past, were to use Sum f1 and Diff. f1 for establishing an amplitude ratio to determine angle of arrival, one would throw away the information available in the Sum f1 and Diff. f1 channels.
There is, however, a further consideration. For instance, to calculate range, if one happens to know that the target is near boresight, then using the Sum f1 and the Sum f2 beams, one can obtain a range estimate. However, one throws away the Diff. f1 and Diff. f2 information.
On the other hand, if one knows that the target is, for instance, about 4 degrees off boresight, then the Sum beams are significantly down in amplitude, for instance, 25 dB down. In this scenario, the Diff. beams are much higher in amplitude than if the target were closer to boresight. Thus, in this scenario it would be more useful to use the Diff. f1 and Diff. f2 beams to find the phase Differences and calculate the range and throw away the Sum f1 and Sum f2 beams.
In the past, there was no convenient way to be able to instantly decide whether to use the Sum f1 and Sum f2 beams or the Diff. f1 and Diff. f2 beams.
This is quite constraining and time consuming because one would have to have first calculated the angle of arrival to make the appropriate estimate. Thus, in prior art systems, in order to resolve this amplitude disparity, one had to first calculate the angle of arrival and then decide whether to keep the Sum beams and throw away the Difference beams, or to keep the Difference beams and throw away the Sum beams.